The function of integrated semiconductor circuits can be influenced by the action of light. At pn junctions, the photon energy radiated in leads to the generation of electron-hole pairs which lead to a current flow in a closed electric circuit. When a voltage is applied to a pn junction in the reverse direction, in the absence of light being radiated in, a leakage current flows, the so-called dark current, which is very small. When light is radiated in, a significantly more intense photocurrent which is dependent on the light intensity is superposed on the dark current.
In many security-relevant IC chips (e.g. security memory ICs), lines on which confidential data are transmitted are arranged in diffusion planes within the semiconductor material, because this affords good protection against physical attacks (e.g. probing by means of emplaced contact needles). Said lines are therefore formed as strip-type doped regions in the semiconductor material. At the outer walls of these diffusion tracks, pn junctions are present toward the substrate or toward a doped well in the substrate, said pn junctions being reverse-biased in normal operation of the circuit. In the event of light incidence, leakage currents occur at said pn junctions, and influence the voltage level on the line, particularly if a dynamic (capacitive) data signal is carried there. Suitable measures can thereupon be initiated if the data transmission has not become indecipherable anyway to the attacker as a result of the voltage drop.
The easiest way of avoiding harmful effects of light attacks on security-relevant circuit parts is if there is a light detector present on the IC chip. However, it is not sufficient for such a light detector to be arranged in a locally delimited fashion since a light beam can also be focused onto security-critical circuits. Therefore, it is necessary to configure the light detector such that it is distributed extensively over the integrated circuit and is ready for use wherever the critical locations of the circuit are arranged. Strictly speaking, an effective light detector should be fitted exactly at the place of the circuit parts to be protected. However, since lines of the wiring of the circuit in different metalization planes are generally situated above the circuit part to be protected, light diffraction effects and scattering effects occur which destroy a very narrowly delimited focusing of the light beam, so that it is sufficient to arrange a light detector structure at least to some extent in the vicinity of the circuit parts to be protected.
When safeguarding an EEPROM memory matrix against light attacks, the problem arises that the lines provided for the read-out of security-critical data cover a comparatively large areal region, so that light detector structures have to be present ubiquitously there. However, light detector structures integrated into the arrangement of the memory cells of an EEPROM memory matrix considerably increase the area requirement occupied by the memory matrix.